Service Manual

Usage Information When you enter the no mtu command, Dell Networking OS reduces the ip mtu value to 1536 bytes. To
return the IP MTU value to the default, use the no ip mtu command.
Starting with Dell Networking OS Release 9.2(0.2), the ip mtu command is not supported to congure the
IP MTU value that is used when the IP packet is fragmented. Instead of having to congure the IP MTU
value, this value is automatically computed by the software when you congure an interface. As a result, the
ip mtu command is not available for conguration. However, you can continue to specify the link MTU value
by using the mtu command.
Compensate for Layer 2 header when conguring link MTU on an Ethernet interface or Dell Networking OS
may not fragment packets. If the packet includes a Layer 2 header, the dierence between the link MTU and
IP MTU (the ip mtu command) must be enough bytes to include for the Layer 2 header.
Link MTU and IP MTU considerations for Port Channels and VLANs are as follows
Port Channels:
• All members must have the same link MTU value and the same IP MTU value.
• The Port Channel link MTU and IP MTU must be less than or equal to the link MTU and IP MTU values
congured on the channel members. For example, if the members have a link MTU of 2100 and an IP
MTU 2000, the Port Channel’s MTU values cannot be higher than 2100 for link MTU or 2000 bytes for
IP MTU.
VLANs:
• All members of a VLAN must have same IP MTU value.
• Members can have dierent Link MTU values. Tagged members must have a link MTU 4 bytes higher
than untagged members to account for the packet tag.
• The VLAN link MTU and IP MTU must be less than or equal to the link MTU and IP MTU values
congured on the VLAN members. For example, the VLAN contains tagged members with Link MTU of
1522 and IP MTU of 1500 and untagged members with Link MTU of 1518 and IP MTU of 1500. The
VLAN’s Link MTU cannot be higher than 1518 bytes and its IP MTU cannot be higher than 1500 bytes.
The following describes the dierence between Link MTU and IP MTU.
Layer 2
Overhead
Dierence between Link MTU and IP MTU
Ethernet
(untagged)
18 bytes
VLAN Tag Tag 22 bytes
Untagged Packet
with VLAN-Stack
Header
22 bytes
Tagged Packet with
VLAN-Stack
Header
26 bytes
Related Commands mtu — sets the link MTU for an Ethernet interface.
IPv4 Routing
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